Magnetic Fields in Galaxies

The origin and evolution of magnetic fields in the Universe is a cosmological problem. Although exotic mechanisms for magneotgenesis cannot be ruled out, galactic magnetic fields could have been seeded by magnetic fields from stars and accretion disks, and must be continuously regenerated due to the ongoing replacement of the interstellar medium. Unlike stellar dynamos, galactic dynamos operate in a multicomponent gas at low collisionality and high magnetic Prandtl number. Their background turbulence is highly compressible, the plasma β ~ 1, and there has been time for only a few large exponentiation times at large scale over cosmic time. Points of similarity include the importance of magnetic buoyancy, the large range of turbulent scales and tiny microscopic scales, and the coupling between the magnetic field and certain properties of the flow. Understanding the origin and maintenance of the large scale galactic magnetic field is the most challenging aspect of the problem.